The present invention relates generally to Boron Neutron Capture Therapy, and, more specifically, to the generation of low energy epithermal neutrons therefor.
Energetic epithermal neutron enable penetration to the site of the tumor. Achieving a suitable neutron energy spectrum is very important for effective treatment. If the neutron energy spectrum is too low, their penetration depth into tissue is too small to reach the site of the tumor, if too energetic, the radiation dose to normal tissue is excessive.
BNCT treatment effectiveness is being experimentally investigated using nuclear reactors as the source of neutrons. In the U.S., several patients have been treated at Brookhaven Medical Research Reactor (BMRR), located at Brookhaven National Laboratory. Leakage neutrons from the core are moderated and collimated to produce a suitable beam at the external treatment port.
Reactors have very low neutron utilization efficiencies. Typically, only about 10.sup.-6 of the neutrons that are released in the core are actually available at the treatment port. This is a result of the inherent dimensional constraints imposed by criticality, and the relatively long distances required to slowdown high energy neutrons using conventional moderators. Gamma shielding requirements are also a contributing factor. As a result, in the BMRR, for example, the treatment port is located at a distance of 177 centimeters from the center of the core. In the MURR (Missouri University Research Reactor) BNCT design, the treatment port is 310 centimeters from the center of the core.
As a result of this very low neutron utilization efficiency, a reactor-based neutron source for BNCT requires high operating power, on the order of several megawatts, and is a large, very expensive, one of a kind facility with a limited capability to treat large numbers of patients.
In contrast, accelerator-based neutron sources for BNCT appear to have very attractive features, as compared to reactor based neutron sources: much lower facility cost, greatly reduced residual radioactivity, much lower operating power, greatly reduced safety concerns, and a better neutron energy spectrum for treatment.
Compared to reactor-based BNCT facilities, accelerator-based facilities could be located at a much larger number of sites, enabling many more patients to be treated.
Various concepts for accelerator-based BNCT systems have been proposed in which a particle beam interacts with a target to generate neutrons. Depending on the particular concept, the nuclear reaction involved can be a (p, n) reaction, a H.sup.3 (d, n) He.sup.4 reaction, and so forth.
A particularly promising approach is the proton beam--lithium target concept, in which a low energy proton beam (about 2 MeV) strikes a lithium target, generating neutrons by the (p, n) reaction. Its attractive features include:
Relatively high neutron yield per proton (about 10.sup.-4); PA1 Low maximum energy of generated neutrons; PA1 Simple, low energy proton accelerator; PA1 Simple, readily cooled target; and PA1 Minimal shielding and residual radioactivity.
A number of design studies of the proton beam--lithium target concept have been carried out, including the use of a radio frequency quadrupole (RFQ) linac to accelerate protons to strike a lithium target with an energy above the 1.8 MeV production threshold for the .sup.7 Li(p,n).sup.7 Be reaction. These previous studies, while they show that the concept is feasible, end up requiring the proton beam current to be in the range of 50-100 milliamps in order to achieve adequate neutron flux at the treatment port.
Accelerators for producing beam currents at this level are technically challenging, and costly as well. In addition, the target generated neutron energy spectrum typically has a substantial fast neutron component that would cause objectionable radiation dose in normal, noncancerous tissue. The gamma dose to normal tissue is also significant. Finally, cooling of the accelerator targets at the required power levels is difficult.
In these previous designs, the high energy neutrons generated by the target/proton interactions are degraded to the treatment regime, i.e., on the order of 10 keV in energy, by scattering collisions with a suitable moderator (e.g., BeO, Al.sub.2 O.sub.3, etc) With such materials, to achieve the requisite energy degradation needed for a useful energy spectrum, the target must be located at some distance from the patient treatment zone. Consequently, for such systems, the neutron utilization efficiency, that is, the ratio of the rate at which useful neutrons are introduced into the patient treatment zone to the rate at which neutrons are generated by proton/target interactions, is typically in the range of 0.1 to 0.5 percent. That is, only 1/1000th to the 1/200th of the neutrons in the target actually are available for use in the patient treatment zone.
However, such efficiencies are still orders of magnitude greater than those achieved by medical reactor systems. Because of the inherently much greater distance between the neutron generating reactor core and the patient treatment zone, due to the inherent dimensional constraints imposed by criticality and the shielding requirements, the neutron utilization efficiency for medical reactors is on the order of 10.sup.-6. Thus for medical reactors, only about one millionth of the generated neutrons actually are available for use in the patient treatment zone.
The accelerator-based proton beam-lithium target approach still has unsolved critical problems including high proton beam current requirements, excess energy neutrons, and cooling of the lithium target which has a low melting temperature. An alternate target being considered is beryllium which has a higher melting point than lithium, is easier to cool, and has been used successfully in clinical fast neutron therapy facilities. The neutron production threshold for protons impinging on a beryllium target is 2.2 MeV and the yield becomes comparable to a lithium target yield at about 4 MeV. However, using 4 MeV protons and a beryllium target produces even more energetic neutrons than the system described above, and therefore requires suitable moderation.
Accordingly, it is desirable to generate low energy epithermal neutrons for BNCT in an accelerator-based apparatus having relatively low proton beam current and suitable cooling of the target.